Avian Assemblage Structure Research Articles

Disentangling an avian assemblages’ evolutionary and functional history in a Chihuahuan desert city

Urban green spaces have been shown to be important hotspots of biodiversity in cities of temperate and humid/semihumid tropical ecoregions. Nonetheless, whether this pattern applies to urban ecosystems of desert environments has been rarely studied. Temperature, precipitation, vegetation complexity, human density, and presence of invasive species could act as urban filters limiting the incidence of desert-adapted species into cities. Such effects could be reshaping biotic communities, favoring habitat generalist species in human-dominated environments. In this study, we examined the phylogenetic and functional structure of avian assemblages in a Chihuahuan desert city and its surroundings to infer the processes underlying community assembly. We used phylogenetic comparative methods to test the hypothesis whether there is an underlying pattern determining which desert-adapted species penetrate or tolerate a novel urban ecosystem. We also performed a regression approach to determine which environmental and anthropogenic variables may be associated with these metrics. We found that urban green spaces present more evolutionary and functional diversity (based on the proportion of total tree branch length) than agricultural fields and desert scrub, although not statistically significant. On the other hand, based on the mean branch length distance among sample taxa, we observed clustered communities suggesting environmental filtering. Most continuous functional traits presented a low and significant phylogenetic signal, but nearly all binary traits were conserved across phylogeny. Phylogenetic predisposition to be a habitat generalist is present in the surveyed avian assemblages. Our regression analysis indicates that invasive bird species richness was negatively correlated with net relatedness index (NRI) and functional net relatedness index (FNRI), while functional diversity metrics were influenced by temperature and precipitation.

Effects of Urbanization on Native Bird Species in Three Southwestern US Cities

Urbanization presents novel challenges to native species by altering both the biotic and abiotic environments. The rapid pace of declines in species diversity and ecosystem services makes generalizations imperative. Studies have attempted to make generalizations about how species with similar traits respond to urbanization, although the results of such studies are geographically idiosyncratic. Here, we present a comparative study in three US cities: Fresno, California; Tucson, Arizona; and Phoenix, Arizona in an attempt to examine consistency in how urbanization affects native bird assemblages. Using presence-absence data to define regional and urban species pools, we tested for whether the urban avian assemblage is a random subset of the regional avian assemblages on the basis of both traits and phylogeny, and whether or not urbanization causes biotic homogenization among avian assemblages. We found little evidence for non-random trait shifts, with only distributions of diet guild, migratory status, and main habitat showing any significant change, and no evidence for non-random phylogenetic patterns in urban avian assemblages. We did however find some evidence for neutral processes in species’ occupancy of urban habitats. Species in the urban species pools have a higher median reporting frequency than species in the regional species pools in all three cities, although this difference is statistically significant in only one city. Cluster analyses show that levels of biotic homogenization are more severe in spring than in winter. The results presented here indicate that while urban avian assemblage structure may be determined by species’ traits, which may possess phylogenetic signal, simple occurrence in an urban area is likely due to random or neutral processes. The seasonality of homogenization has not to our knowledge been previously reported. We propose that the largely similar results from the three cities in this study result from structural similarities in the matrix habitats, and that the nature of matrix habitat and context of urbanization needs to be considered in future studies in order to resolve existing inconsistencies.

Land use types influenced avian assemblage structure in a forest-agriculture landscape in Ghana.

The conservation of biodiversity within tropical forest regions does not lie only in the maintenance of natural forest areas, but on conservation strategies directed toward agricultural land types within which they are embedded. This study investigated variations in bird assemblages of different functional groups of forest‐dependent birds in three agricultural land types, relative to distance from the interior of 34 tropical forest patches of varying sizes. Point counts were used to sample birds at each study site visited. Data from counts were used to estimate species richness, species evenness, and Simpson's diversity of birds. Mean species richness, evenness, and diversity were modeled as responses and as a function of agricultural land type, distance from the forest interior and three site‐scale vegetation covariates (density of large trees, fruiting trees, and patch size) using generalized linear mixed‐effect models. Mean observed species richness of birds varied significantly within habitat types. Mean observed species richness was highest in forest interior sites while sites located in farm centers recorded the lowest mean species richness. Species richness of forest specialists was strongly influenced by the type of agricultural land use. Fallow lands, density of large trees, and patch size strongly positively influenced forest specialists. Insectivorous and frugivorous birds were more species‐rich in fallow lands while monoculture plantations favored nectarivorous birds. Our results suggest that poor agricultural practices can lead to population declines of forest‐dependent birds particularly specialist species. Conservation actions should include proper land use management that ensures heterogeneity through retention of native tree species on farms in tropical forest‐agriculture landscapes.

Structure of avian assemblages in Zambezian Baikiaea woodlands, northern Namibia

AbstractThe Zambezian Baikiaea woodlands fall entirely within the Kalahari Woodland biome, a subset of savanna. In 2015, the line transect method was employed to study avian assemblages associated with Zambezian Baikiaea. In total, 10 transects were designed along a straight 12 km long line, so that each transect was 1.2 km long. A total of 88 species represented by 1190 pairs were recorded. On average, there were 42.9 species per transect. The overall density was 35.8 pairs/km, ranging from 29.2 to 44.2 on particular transects. Overall, Streptopelia capicola was the only dominant species. It was also the only species recorded as dominant on all the transects. The following other species were recorded as dominant: Cercotrichas leucophrys, Laniarius aethiopicus, Turtur chalcospilos, Dicrurus adsimilis, Lamprotornis nitens, Upupa africana, Apalis flavida, Ploceus velatus, Tchagra australis and Numida meleagris. The proportion of dominant species (cumulative dominance) varied from 18.9 to 37.7 (x = 28.5; n =...

Structure of avian assemblages in grasslands associated with cattle ranching and soybean agriculture in the Uruguayan savanna ecoregion of Brazil and Uruguay

Conversion of grasslands into crops is a major factor leading to the decline of grassland birds. Cattle ranching represents another disturbance to natural grasslands, but may be less detrimental to grassland birds. We studied the diversity, density, and composition of bird species in Brazilian and Uruguayan grasslands under two different land use types: cattle ranching on seminatural grasslands, and soybean fields with interspersed patches of grassland. Cattle sites had higher species richness (n = 75 species) than soybean sites (n = 57 species). Most birds showed higher densities in cattle sites, but some common and habitat-generalist species were more abundant in soybean sites. Species composition did not differ significantly with land use. The generalist Eared Dove (Zenaida auriculata), however, was strongly associated with soybean sites. Among species of conservation interest, either regionally or globally, all had higher densities in cattle sites, highlighting the importance of maintaining these ranching areas. The persistence of grassland birds in soybean fields may be related to the presence of seminatural grassland patches within soybean crops.

Relative roles of grey squirrels, supplementary feeding, and habitat in shaping urban bird assemblages.

Non-native species are frequently considered to influence urban assemblages. The grey squirrel Sciurus carolinensis is one such species that is widespread in the UK and is starting to spread across Europe; it predates birds’ nests and can compete with birds for supplementary food. Using distance sampling across the urbanisation intensity gradient in Sheffield (UK) we test whether urban grey squirrels influence avian species richness and density through nest predation and competition for supplementary food sources. We also assess how urban bird assemblages respond to supplementary feeding. We find that grey squirrels slightly reduced the abundance of breeding bird species most sensitive to squirrel nest predation by reducing the beneficial impact of woodland cover. There was no evidence that grey squirrel presence altered relationships between supplementary feeding and avian assemblage structure. This may be because, somewhat surprisingly, supplementary feeding was not associated with the richness or density of wintering bird assemblages. These associations were positive during the summer, supporting advocacy to feed birds during the breeding season and not just winter, but explanatory capacity was limited. The amount of green space and its quality, assessed as canopy cover, had a stronger influence on avian species richness and population size than the presence of grey squirrels and supplementary feeding stations. Urban bird populations are thus more likely to benefit from investment in improving the availability of high quality habitats than controlling squirrel populations or increased investment in supplementary feeding.

Incidence of competitors and landscape structure as predictors of woodland-dependent birds

Globally, modification of landscapes for agriculture has had a strong influence on the distribution and abundance of biota. In particular, woodland-dependent birds are under threat across agricultural landscapes in Britain, North America and Australia, with their decline and extirpation attributed to the loss and fragmentation of habitat. Other native species have become over-abundant in response to anthropogenic landscape change and have strong interactive effects on avian assemblage structure. In eastern Australia, the hyper-aggressive noisy miner (Manorina melanocephala) often dominates woodlands in agricultural landscapes through interspecific competition, resulting in declines of species richness of woodland-dependent birds. We aimed to determine the relative influence and importance of interspecific competition, in situ habitat structure and landscape structure for woodland-dependent bird species at the landscape level. We recorded species-specific landscape incidence of woodland-dependent birds in 24 agricultural-woodland mosaics (25 km2) in southern Queensland, Australia. We selected extensively cleared landscapes (10–23 % woodland cover) where fragmentation effects are expected to be greatest. We applied generalised linear models and hierarchical partitioning to quantify the relative importance of the landscape-level incidence of the noisy miner, mistletoe abundance, shrub cover, woodland extent, woodland subdivision and land-use intensity for the incidence of 46 species of woodland birds at the landscape-scale. The landscape-level incidence of the noisy miner was the most important explanatory variable across the assemblage. Both in situ habitat structure and landscape structure were of secondary importance to interspecific aggression, although previous research suggests that the increasing incidence of the noisy miner in fragmented agricultural landscapes is itself a consequence of anthropogenic changes to landscape structure. Species’ responses to fragmentation varied from positive to negative, but complex habitat structure had a consistently positive effect, suggesting in situ restoration of degraded habitats could be a conservation priority. Landscape wide conservation of woodland-dependent bird populations in agricultural landscapes may be more effective if direct management of noisy miner populations is employed, given the strong negative influence of this species on the incidence of woodland-dependent birds among landscapes.

Avian functional group responses to rainfall across four vegetation types in theSimpsonDesert, centralAustralia

AbstractTemporal and spatial variability in rainfall and subsequent resource flows often drive large changes in bird demography and the structure of avian assemblages in arid regions. Here, we describe broad avian functional group responses to rainfall across four vegetation classes in theSimpsonDesert, centralAustralia, over a period when both dry and wet conditions prevailed. Birds were surveyed in eucalypt woodlands, gidyea woodlands, mixedAcacia(mulga) shrublands and spinifex‐dominated hummock grasslands in this region between 2006 and 2008 using a spatially nested hierarchical sampling design with fixed radius point counts as replicates. Predictably, the composition of bird communities varied across vegetation classes as well as over time. Generalist feeders and insectivorous birds dominated assemblages during dry periods, whereas granivorous species were most abundant and widespread following heavy rains. Carnivores and specialist nectarivores responded variably throughout the sampling period regardless of habitat. The dynamics of granivores were especially interesting; large numbers of nomadic species moved into the study area in response to rainfall, highlighting a key role for this group in shaping community structure. This study represents the first attempt to investigate avian functional group dynamics across differing vegetation types in a variable but low productivity landscape. The findings confirm that community composition is spatially and temporally dynamic, and prompt further investigation into the roles that habitat and resource limitation play in defining bird communities in aridAustralia.

Sap Consumption by the White-fronted Woodpecker and Its Role in Avian Assemblage Structure in Dry Forests

Abstract. The White-fronted Woodpecker (Melanerpes cactorum) drills holes in branches and trunks to feed on sap flows, providing an energy-rich food resource for other birds. Here we describe ecological and behavioral traits of the White-fronted Woodpecker related to its sap-feeding habits in the semiarid Chaco of Argentina and explore the structure of the avian assemblage in relation to the sap resource. Sap consumption by the White-fronted Woodpecker and other sap-feeding species was strongly seasonal and positively associated with periods of resource scarcity. The White-fronted Woodpecker actively defended the sap wells from smaller birds. Specialist and facultative nectarivores that assimilate sucrose at a high rate represented an important proportion of sap-feeding birds. In this system of woodpecker, sap, and other sap-feeding species, each species' consumption depends on its physiological and behavioral characteristics as well as on the availability of other food in the surrounding environment.

Avian species-assemblage structure and indicator bird species of mangroves in the Australian monsoon tropics

Mangroves are threatened worldwide yet they host diverse avian assemblages, especially in Australia. We examined the diversity, density and habitat use of birds in northern Australian mangroves to determine the influence of habitat patch-size and floristic composition on the structure of mangrove bird assemblages. Birds were surveyed using line transects in 13 mangrove patches. A total of 70 species was encountered, with a mean density of 11.7 individuals ha−1 including 11 of 12 known mangrove-dependent species. Many species were strongly associated with, and indicative of, a specific mangrove plant zone, emphasising the importance of the number of zones to bird diversity at a site. Of six functional guilds, the insectivores were most dominant, followed by nectarivores. Accordingly, avian assemblage structure was influenced by mangrove flowering phenology. Large mangrove patches supported fewer species than many smaller patches of equivalent combined area, and species richness was independent of area. In addition, there was no density compensation or a density—area relationship, implying that assemblages are not saturated with species and species interactions do not determine assemblage structure. In order of increasing importance, avian assemblage structure in mangroves is determined by the type and diversity of mangrove zones, the timing of mangrove flowering and the nature of the matrix surrounding mangroves.

Despotic, high‐impact species and the subcontinental scale control of avian assemblage structure

Some species have disproportionate influence on assemblage structure, given their numbers or biomass. Most examples of such "strong interactors" come from small-scale experiments or from observations of the effects of invasive species. There is evidence that entire avian assemblages in open woodlands can be influenced strongly by individual species over very large areas in eastern Australia, with small-bodied species (< 50 g) being adversely affected. We used data from repeated surveys in 371 sites in seven districts across a region from Victoria to Queensland (> 2000 km). A series of linked Bayesian models was used to identify large-bodied (> or = 50 g) bird species that were associated with changes in occurrence and abundance of small-bodied species. One native species, the Noisy Miner (Manorina melanocephala; family Meliphagidae), was objectively identified as the sole large-bodied species having similar detrimental effects in all districts, depressing occurrence of 57 of 71 small-bodied species. Adverse effects on abundances of small-bodied species were profound when the Noisy Miner occurred with mean site abundances > or = 1.6 birds/2 ha. The Noisy Miner may be the first species to have been shown to influence whole-of-avifauna assemblage structure through despotic aggressiveness over subcontinental scales. These substantial shifts in occurrence rates and abundances of small-bodied species flow on to alter species abundance distributions of entire assemblages over much of eastern Australia.

Seasonal and Multiannual Patterns in Avian Assemblage Structure and Composition in Northern Chilean Thorn-Scrub

Abstract. The species composition of the Chilean avifauna is well-defined taxonomically but is not well known ecologically. We sampled avian communities at a biosphere reserve in coastal north-central Chile in three seasons over six years (18 surveys total) and characterized them in terms of community structure and composition. The avifauna (S = 56 species) was dominated by insectivores (S = 20), carnivores (S = 14), and granivores (S = 13), with lesser contributions by omnivores (S = 5), nectarivores (S = 2), folivores (S = 1), and one vagrant piscivore. The fauna varied greatly between summer and winter, and in most years the breeding season also was distinct. Eighteen species constituted a core group of residents observed in nearly all surveys, but at least 15 species were nomadic or migratory. Our site supported more insectivorous species in winter but more granivores and omnivores in the breeding season, although this observation may be confounded by species' detectability. The structure of the set o...

Effect of stand age on bird communities in late-successional Macedonian pine forests in Bulgaria

To gain insight into the question of which vegetation characteristics have the most influence on avian assemblages in late-successional forests, the habitat preferences of bird-guilds in old-growth endemic forests of Macedonian pine were studied over 3 years in the Pirin National Park, Bulgaria. Bird–habitat relationships were investigated by comparing vegetation characteristics, and bird species richness, diversity, abundance, and guild structure of birds (determined according to food type, foraging and nesting sites) between mature (60–100 years old) and over-mature (>120 years old) Macedonian pine forest stands. Studied forest age-classes differed mainly by the density, height and diameter of trees, and the amount of dead wood. The first one of these parameters decreased and the latter two parameters increased with the forest succession. The difference in the vegetation structure affected the abundance of bird-guilds and thus, the overall bird abundance and the structure of avian assemblages within Macedonian pine forests. There was no significant difference in bird diversity among studied forest age-classes, but the overall bird abundance increased with forest maturation. Analyzed by study plots, species richness was higher in over-mature forests, but at cluster level, there was no significant difference between mature and over-mature forest age-classes. Half of the studied (insectivorous, hole- and ground-nesters, bark- and canopy-foraging bird species) guilds were more abundant in over-mature forests, while there was no bird-guild exhibiting a preference for mature forest stands. The abundances of bird-guilds were correlated with tree height, diameter at breast height and the amount of dead wood between the studied forest age-classes and this might explain their preferences for over-mature pine forests. Therefore, for future sustainable management of these endemic forests and the conservation of their avifauna, efforts should focus on protecting the remaining native old-growth forest stands and the importance of the structure of Macedonian pine forests on their bird assemblages should be considered in forestry practices.

Avian assemblage structure and domestic cat densities in urban environments

ABSTRACTWhile there is intense debate regarding the impact of domestic cat populations on wildlife, its resolution is hindered by the lack of quite basic information. Domestic cats are generalist and obligate predators that receive supplementary food, and their population density reflects that of humans more than the density of their prey. In such a predator–prey system there is the potential for cat populations to have negative impacts on avian assemblages, which may be indicated by negative correlations between cat density and avian species richness and density. Here we report on the nature of such correlations across urban areas in Britain both for groups of species classified regarding their vulnerability to cat predation and individual species. Taking the availability of green space into account, we find negative relationships between cat densities and the number of bird species breeding in urban 1 km × 1 km squares. These relationships are particularly strong among groups of species that are vulnerable to cat predation. We find positive correlations between cat and avian densities; these have low explanatory power and shallow slopes among the species groups that are particularly vulnerable to cat predation. Evidence that the densities of individual species that are vulnerable to cat predation are negatively correlated with cat densities is equivocal, with at least half the species showing no marked pattern, and the remainder exhibiting contrasting patterns. Our results appear not to be confounded by the density of nest‐predating corvids (carrion crow, magpie, and jay), as the density of these species was not strongly negatively correlated with avian species richness or density. The general lack of marked negative correlations between cat and avian densities at our focal spatial scale may be a consequence of consistently high cat densities in our study areas (minimum density is 132 cats per square kilometre), and thus uniformly high impacts of cat populations on urban avian assemblages.

Changes in the diversity structure of avian assemblages in North America

AbstractAim To determine the major patterns of change in avian diversity structure over space and time at a local resolution and continental extent in non‐urban areas in North America.Location The contiguous United States and southern Canada.Methods We used 1673 North American Breeding Bird Survey (BBS) routes containing 547 terrestrial and aquatic species to estimate four diversity components: species richness, individual abundance, taxonomic distinctness and species evenness. We implemented three levels of analysis to examine changes in diversity structure on a yearly basis from 1968 to 2003: (1) a canonical analysis of discriminance, (2) a univariate analysis across BBS routes, and (3) a univariate analysis at individual BBS routes. We estimated changes in similarity in species composition over time between 470,730 BBS route pairs. We also estimated the level of human activity at BBS routes using three spatial anthropogenic databases.Results BBS routes were located in non‐urban areas in association with low nighttime light activity and moderately low human population densities. The analysis of diversity structure indicated the presence of two independent patterns: (1) a temporally consistent pattern defined by increasing species richness (12% increase from 1968 to 2003) associated with limited gains in taxonomic distinctness, and (2) an association between species abundance and evenness related to variability in abundance associated with the most abundant species. The similarity analysis indicated that BBS routes located closer to the Atlantic and the Pacific coasts of the United States experienced the strongest patterns of homogenization of species composition.Main conclusions Our results suggest that avian diversity structure has changed at the local scale in non‐urban areas in North America. However, there was no evidence for a consistent continent‐wide pattern. Instead, the evidence pointed to the presence of regional factors influencing diversity patterns. This study provides a foundation for more detailed investigations of the spatiotemporal and taxonomic details of these general patterns.

Foraging behaviour of birds in an arid sand-dune scrubland in Argentina

Studies of the foraging behaviour of birds allow exploration of the mechanisms that structure species assemblages. The major objective of this study was to describe relationships among various components of foraging behaviour and whether changes in those relationships reflected seasonal changes in assemblage structure of birds from an arid scrubland of the Monte Desert, Argentina. Foraging behaviour was described in terms of attack manoeuvres, food substrate (i.e. substrate from which food was taken), foraging site (i.e. the substrate and height from where birds launched the attack), and plant species used. Between-species differences in attack manoeuvres, food substrate and foraging site were interrelated, the association being strongest between foraging sites and food substrates. During the non-breeding season, foraging sites and food substrates differentiated species. The most important changes between seasons were related to the arrival in summer of aerial-foraging tyrant flycatchers, a functional group absent during the non-breeding season. Foraging sites explained most of the differences among species groups, suggesting that habitat heterogeneity and structural complexity of vegetation are important environmental variables that determine the avian assemblage structure. Moreover, temporal changes in food availability, mediated by strong seasonality in climate, were important factors that were correlated with compositional and structural variability in functional groups of birds (i.e. granivores-insectivores, surface insectivores and aerial insectivores).

Resources and global avian assemblage structure in forests

AbstractExplaining spatial variation in a number of bird species, particularly from temperate to tropical regions, has been a longstanding challenge. We test at a global scale whether species‐rich forest assemblages are associated with division of a larger resource pool, a finer division of that pool, or some combination of the two. Species richness increases with increasing assemblage abundance, biomass and energy use. As assemblage abundance, biomass and energy use increase with increasing energy availability, and as per species numbers of individuals, biomass and energy use do not decrease with increasing energy availability, we provide direct evidence that the avian species–energy relationship in forests is associated foremost with an increase in the size of the resource pool and not with a finer level of its subdivision.